1. Field of the Invention
The present invention relates to a tool axis direction calculation method used in an automatic programming apparatus for creating NC data (machining program) for cutting a sculptured surface of a metal mold, and more specifically, to a tool axis direction calculation method of determining a direction vector of a tool axis in a side cut carried out by a 5-axes numerically controlled machine tool.
2. Description of the Related Art
Automatic programming apparatuses for creating NC data for cutting a sculptured surface of a metal mold are provided with a software for creating various NC data, and this software can simultaneously determine offset data for a tool.
Also, 5-axes numerically controlled machine tools having B- and C-axes for inclining a tool axis in addition to the usual X-, Y- and Z-axes, are available and can cut a side of a workpiece by using a flat end mill.
When making this side cut, a direction vector of a tool axis is generally determined by the following calculation method:
(a) determining dividing points corresponding to the upper and lower surfaces of a curved surface to be cut;
(b) determining respective normal vectors at these dividing points;
(c) determining points obtained by offsetting the radius of a tool to the directions of the respective normal vectors; and
(d) creating a vector by connecting these points and using the thus created vector as the direction vector of the tool axis.
Nevertheless, this method has the following drawbacks:
First, the direction of the normal vector of the upper surface of a curved surface to be cut does not always coincide with the direction of the normal vector of the lower surface thereof, and thus the direction vector of a tool axis may not be parallel to the generating line of the curved surface and may be twisted.
Second, the tool axis may not be on the line segment (generating line of the curved surface) obtained by connecting the points corresponding to the upper and lower surfaces, respectively, of the curved surface to be cut. More specifically, the tool axis is on the generating line only when the normal vector of the upper surface is equal to the normal vector of the lower surface.
As a result, a sculptured surface to be determined is different from an actual machined surface, i.e., a workpiece is bittin or partially not cut.